Abstract

Tapered optical fibers (TOFs) modified by 400 nm-diameter silica nanospheres have been experimentally and theoretically investigated. Responses of the modified TOFs (MTOFs) to methylene blue (MB) solution and surrounding refractive index (RI) have been compared. The higher-order modes are effectively filtered by the silica nanospheres, which is visualized by the interferometric output spectrum of the MTOF. The higher-order mode filtering makes the MTOF respond to MB solution with reduced sensitivity, which endows the MTOF with the potential for distinguishing the RI property from the absorption property of the molecules under detection.

Normalized spectral responses of (a) TOF and MTOF with a 3 μm-thick waist immersed in water, (b) a TOF with a 5 μm-thick waist immersed in glycerin aqueous solutions with different refractive indices, and (c) a TOF with varying waist diameter immersed in water. D, waist diameter; unit, μm. (d) Absorbancy of TOF and MTOF with respect to the MB concentration.

(a) Schematic representing the scattering of modes by a silica nanosphere. (b) Calculated transmission (PRj/PSj) of mode as a function of the number of silica nanospheres. (c) Calculated (bottom red curve) oscillatory spectral response due to beating between HE11 and HE12 modes, and the experimental result (top blue curve) shown in Fig. 3(a) is plotted again for comparison.